In a multi-cylinder engine, each fuel injection valve has dispersion in fuel injection characteristic thereof, which may cause an unstable rotation of a crankshaft of the engine. A deviation amount between the fuel injection characteristic of each fuel injection valve and a reference fuel injection characteristic is learned in order to uniform the rotation speed of the crankshaft which is obtained by each fuel injection in each cylinder. DE-19527218B4 shows such a control system.
The deviation amount is varied according to the fuel pressure supplied to the fuel injection valve. JP-2003-254139A shows that the deviation amount is learned with respect to a plurality of regions which are defined by the fuel pressure. The deviation amount is learned according to the fuel pressure, so that the fuel injection valve is appropriately operated to compensate the deviation amount. When the fuel injection valve is operated with the deviation amount, an interpolating process is usually used. In the interpolating process, a representative point is defined with respect to each region. In a case that the representative point is not consistent with the actual fuel pressure, a deviation amount corresponding to the actual fuel pressure is calculated by interpolating process by use of a deviation amount at a plurality of representative points adjacent to the actual fuel pressure. Thereby, the fuel injection valve is appropriately operated in such a manner as to compensate the actual deviation.
However, in a case that a representative point exists in which the deviation amount has not been learned, the interpolating process cannot be conducted appropriately. Furthermore, even if the learning is conducted, the appropriate deviation amount cannot be learned by conducting the interpolating process only once. Such a problem may arise to a control system in which a deviation amount of the operational characteristic of the actuator, which is used for fuel injection control, relative to the reference amount is learned with respect to each representative point defined by the parameters used for fuel injection control calculation.